 // Package provides a shifted fibonacci encoding of unsigned integers.
//
// http://en.wikipedia.org/wiki/Fibonacci_coding maps positive integers as
// 1 - 11, 2 - 011, 3 - 0011, 4 - 1011, 5 - 00011
//
// Incrementing input by one to allow for zero gives
// 0 - 11, 1 - 011, 2 - 0011, 3 - 1011, 4 - 00011
//
// The codes are reversed so that they are easily stored in uints,
// effectively avoiding the need to store the number of leading zeroes
// 0 - 11, 1 - 110, 2 - 1100, 3 - 1101, 4 - 11000
package fibonacci

import (
"io"
)

// Alias type with methods for encoding and decoding integers
type Numbers []uint64

var (
// Used for decoding byte values
codec Numbers
// Used for encoding byte values
// The lower 16 bits store the encoded value itself
// while the remaining upper ones store its length
lookup uint32
)

func init() {
codec = New(16)
for i := uint64(0); i < 256; i++ {
val, len := codec.Code(i)
lookup[i] |= uint32(val)
lookup[i] |= uint32(len) << 16
}
}

// Returns a slice with fibonacci numbers up to the given length
func New(size int) Numbers {
var fibs Numbers = make(Numbers, size)
copy(fibs, []uint64{1, 1})
for i := 2; i < size; i++ {
fibs[i] = fibs[i-1] + fibs[i-2]
}
return fibs
}

// Returns a fibonacci code for an integer as specified in the package's doc.
func (f Numbers) Code(value uint64) (result uint64, length byte) {
// Increment to encode zero as one
value++

// Find the nearest fibonacci number
for f[length] <= value {
length++
}

// Leading bit that signals the start of a fibonacci-encoded integer
result |= 1

// Find the Zeckendorf's representation by raising a bit for each
// fibonacci number that is less or equal to the difference
// between the value and the previous such number
for i := length - 1; i >= 1; i-- {
result <<= 1
if f[i] <= value {
result |= 1
value -= f[i]
}
}
return
}

// Returns an integer from a fibonacci code as specified in the package's doc.
func (f Numbers) Decode(value uint64) (result uint64, length byte) {
length = 1
// Loop until the lowest two bits are both raised
for (value & 3) != 3 {
// Add the fibonacci number for the current bit if it is raised
if (value & 1) == 1 {
result += f[length]

// We know that the next bit cannot be raised by Zeckendorf's theorem
value >>= 2
length += 2
continue
}

value >>= 1
length++
}
return result + f[length] - 1, length + 1
}

// Returns a fibonacci encoder over the provided io.Writer
func Encoder(target io.Writer) io.Writer {
var enc encoder
enc.target = target
return &enc
}

type encoder struct {
target    io.Writer
buffer    byte
remaining byte
length    byte
}

// Implements io.Writer
func (e *encoder) Write(input []byte) (int, error) {
var (
total int
err   error
)

// Flush on a nil slice
if input == nil {
_, err = e.target.Write([]byte{byte(e.remaining)})
return 0, err
}

for _, currentByte := range input {
// Get the fibonacci code and bit length for the current byte
enc, len := uint16(lookup[currentByte]), byte(lookup[currentByte]>>16)

// Add current bits to higher positions
e.remaining |= byte(enc << e.length)

// maximum length of added bits to e.remaining

// Shift the the encoded value and account for its length
e.length += len

// Not enough bits to write
if e.length < 8 {
// Account for the processed input byte
total++

continue
}

// Clearing e.length is not necessary as it will be overwritten later

// Stage the complete byte for writing
buffer := e.buffer[:1]
buffer = byte(e.remaining)

// Stage every full byte from the encoded value for writing
//
// The bitlength of the largest encoded byte value, 255, is 13.
// Even with 7 bits already in the buffer this leaves [7+1], 
// and 4 bits remaining => a single if is enough instead of a for.
//
// 128 is [1000 0000] in binary. Any value equal or greater than it
// will be atleast 8 bits in length
if enc >= 128 {
buffer = append(buffer, byte(enc))
enc >>= 8
len -= 8
}

// Store the remaining bits
e.remaining, e.length = byte(enc), len

// Write the staged bytes
_, err = e.target.Write(buffer)

// Abort write on error
if err != nil {
break
}

// Account for the processed input byte
total++
}
}

// Returns a fibonacci decoder over the provided io.Reader
var dec decoder
dec.source = source
return &dec
}

type decoder struct {
buffer uint64
at     byte
}

func (d *decoder) Read(output []byte) (int, error) {
var (
total int
err   error
)

start:
// While we have suitable buffered data and enough output space
for (len(output) > 0) && ((d.buffer & (d.buffer >> 1)) > 0) {
val, len := codec.Decode(d.buffer)

// Store the decoded byte
output = byte(val)

// Advance the internal and output buffers
output = output[1:]
d.buffer >>= len
d.at -= len

// Account for the processed output byte
total++
}

// Termination condition
if len(output) == 0 || err != nil {
}

// We need to limit the output's size else we could end up with a lot of small values
// that fit neither in the output slice nor in the internal buffer
//
// (63 is [0011 1111] in binary, xor is a substraction and right shift a division)
free := int((63 ^ d.at) >> 3)
if free > len(output) {
free = len(output)
}

// Read data and transfer to the internal buffer